The invention relates to machine elements and mechanisms but more particularly, the invention relates to an improved sprocket for use with a toothed, power transmission belt, where the sprocket is at least partially formed with a plurality of a plastic-fiber composite material.
There are several types of power transmission wheels such as gear wheels, sprocket wheels, and pulley wheels which have a power transmission surface or layer that is formed from a composite material of plastic and embedded fibers. Some power transmission wheels are formed totally of a composite material while others may include a metallic insert which forms a hub portion and where the composite material is used on a rim portion. An example of such composite materials include fabrics impregnated with a phenolic resin. Another composite material that has the advantage of being capable of being molded by a thermal plastic injection molding is a composite material of nylon reinforced with milled glass fibers having a length of 1/16 in. or less. While a thermal plastic injected nylon material containing a 40 weight percent of nylon fiber is effective for use in many power transmission wheel applications, such composite materials are not suited for long life in some gritty and abrasive environment applications where it is customary to use hard materials such as cast iron, steel, powdered metal or aluminum coated with a hard, chrome oxide plating. A characteristic of composites with glass fibers used in some gritty environments is that in time, the plastic matrix is worn away exposing end portions of fiber juxtaposed the power transmission surface of the power transmission wheel such as a sprocket that may lead to early failure of a component part of a drive system such as a power transmission belt. The exposed fibers being substantially harder than the belt material and pointed, abraid the surface of the belt teeth and cause early belt failure.
Examples of various types of plastic gears and problems associated therewith are summarized in the Jan. 21, 1988 and Oct. 13, 1988 issues of Machine Design.